What are snubber & MOV circuits for?

Snubber circuits are used to control voltage transients that could falsely turn on or damage a thyristor. The object of the snubber is to suppress the voltage transients and ringing that occurs when the thyristor is switched by providing an alternate path for the current flowing through the circuit’s intrinsic leakage inductance.

Snubber circuit

The simplest way of protecting a thyristor from these overvoltage transients is to arrange a resistor and a capacitor connected in series directly in parallel to the thyristor.

Snubber Circuit Diagram

In this circuit the capacitor absorbs the excess transient energy, while the resistor defines the applied dv/dt in conjunction with the external system inductance i.e. the energy is quickly stored in the capacitor then dissipated slowly in the resistor. There are many different combinations of RC values that are capable of providing acceptable protection of the thyristor in its relevant circuit. However, inadequate values can cause an unstable circuit and thyristor damage.

Where do the voltage transients come from?

There are normally voltage transients generated externally to the thyristor circuit and internally. The external transients are generated by the AC supply, coming mainly from the energy stored in the transformer. The internal transients are created by each thyristor commutation, mainly at turn off. While the thyristor is conducting there is inductance in series which generates a high peak reverse voltage when turned off. When the thyristor goes into its blocking state the capacitor suppresses the voltage surge (dv/dt) and the resistor damps the oscillations in the LC circuit as well as limiting the discharge current from the capacitor through the thyristor when it begins conducting again.

Is there any further overvoltage protection?

Some of the best snubber circuits also include voltage limiting MOVs (Metal Oxide Varistors) along with the RC circuit. This can be done by connecting the MOV across the input line as shown for a single phase and a three phase circuit.

Voltage Protected Circuit Diagram

The MOV will then protect the parallel circuit and the load. It will set the maximum input voltage and di/dt through the load while the RC snubber sets the maximum dv/dt and peak voltage across the thyristor. MOVs used in this way must be quite large to prevent damage.

Alternatively a more common approach is to use an MOV across each thyristor. The load impedance limits the surge energy from the line and this allows a smaller MOV to be used. This protects each thyristor by limiting the maximum voltage and snubber discharge di/dt through it.

MOZ Circuit Diagram

Power Products International are experts in effective transient suppression techniques

Power Products International have been manufacturing power electronic assemblies for over 30 years. This experience and knowledge of how to protect semiconductor devices from unavoidable overvoltage transients ensures that the power electronic assembly gives many years of reliable service.

We have our own design of snubber products as well as ranges from Semikron and MOV devices from Zenamics in order to select the most appropriate suppression device for a given application.

Contact us for assistance with thyristor assemblies or any questions regarding effective transient suppression that you may have.

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